Ultraviolet (UV) light in the form of germicidal lamps has been used since the early 1900's to kill the same types of microorganisms that typically cause the same types of problems today. Since then, UV radiation in the short wave or C band range (UVC) has been used in a wide range of germicidal applications to destroy bacteria, mold, yeast and viruses. After World War II, the use of UVC rapidly increased. UVC is generally understood to exist in the 180 nm to 280 nm wave length area. Typical examples included hospitals, beverage production, meat storage and processing plants, bakeries, breweries, pharmaceutical production and animal laboratories; virtually anywhere microbial contamination was of concern. Early UVC strategies primarily consisted of an upper air approach. This method directed a beam across the ceiling of a room.
During the 1950's when tuberculoses infections were on the rise, the use of UVC became a major component in the control and irradiation of TB. It was discovered that by placing UVC lamps in the air handling equipment, they could initially be more effective.
However, certain conditions found within the air handling systems drastically reduced UVC performance. Moving air, especially below 77.degree. F., over the tubes decreased the output and service life of conventional UVC products and thus their ability to destroy viable organisms. The use of UVC and HVAC systems virtually disappeared over the next decade due to the introduction of new drugs, sterilizing cleaners and control procedures combined with the performance problems of UVC lamps and air handling systems (reduced output, short tube life, and high maintenance). In order for UVC to be effective in the "hostile" environment of indoor central air circulating systems (or HVAC systems), a new method of producing effective UV had to be developed.
The ability of ultraviolet light to decompose organic molecules has been known for a long time, but it is only recently that UV cleaning of surfaces has been explored. In 1972, it was discovered that ultraviolet light could clean contaminated surfaces. Plus, it was learned that there exists a predictable nanometer location of absorption of ozone and organic molecules. It was then learned that the combination of ozone and UV could clean surfaces up to two thousand times quicker than one or the other alone. However, from testing it can be seen that the destructive potential of a combination of UVC and ozone for system components is detrimental. The negative side effects of ozone are now known.
In 1972, tests were conducted using a quartz tube filled with oxygen. A medium pressure mercury (Hg) UV source which generated ozone was placed within centimeters of the tube. A several thousand angstrom thick polymer was exposed to this and was depolymerized in less than one hour. The major products of this reaction were water (H.sub.2 O) and carbon dioxide (CO.sub.2). It was discovered that UV (300 nm and below) and oxygen played a major role in depolymerization. In 1974, research concluded that during such cleaning, the partial pressure of O.sub.2 decreased and that of CO.sub.2 and H.sub.2 O increased, suggesting breakdown.
It was also discovered that the absorption coefficient of O.sub.2 increases rapidly below 200 nm with decreasing wave lengths. A 184.9 nm wave length (optimal spectral line for ozone generation) is readily absorbed by oxygen, thus leading to the generation of ozone (O.sub.3). Ozone may be generated at undetectable levels at other wave lengths below 200 nm. Therefore, radiation emission below 200 nm was found undesirable.
Similarly, most organic molecules have a strong absorption band between 200 nm and 300 nm. A wave length of 253.7 nm is useful for exciting and disassociating contaminant molecules. 265 nm was thought to be the optimal spectral line for germicidal effectiveness. The 253.7 nm wave length is not absorbed by O.sub.2, therefore, it does not contribute to ozone generation, but it is absorbed by most organic molecules and by ozone (O.sub.3). Thus, when both wave lengths are present, ozone is continually being formed and destroyed. Unfortunately, previously existing lamps operated between 250 nm and 258 nm, peaking at 254 nm, missing out on the optimal 265 nm goal.
With regard to HVAC systems, biological contaminants are difficult to control because they grow in our moist, indoor environment. The most common strategy is to try to use an effective air system filter to rid indoor air of biological contaminants. While this is an important element of cleaning air, this has its problems. Most filters are inadequate because of the many organisms that pass right on through the filter. Also, any organisms that collect on the filter can form germ colonies that may soon contaminate passing air. Further, if the filter should be too efficient, it blocks the passage of air and creates back pressure, causing the blower to struggle to move air through the system. Furthermore, when the system is turned off, natural temperature differences between the system and indoor air spaces cause convection or back draft flow into the supply ducts (bypassing the filter). This causes contaminants to be pulled back into the duct work, implanting microbes in the air flow duct cavity. These new cultures become added sources of contaminant.
In the past, to try to eliminate the biological contaminants in ducts, a common strategy was to clean the ducts followed by a biocide treatment. But this has its draw backs also. Most biological contaminants return and are active in the treated area within three months. Further, if the system is being treated for severe contamination such as legionela, an acid wash of the coil is common. This is not only expensive, but can shorten the life of the equipment. Furthermore, all biocide used in the ducts are chemical based, leaving potential toxic vapors and chemical pollutants circulating in the system as well. For obvious health reasons, the preferred way to control biological contaminants is through natural, non-polluting strategies.
As indicated above, the effective killing power of UV seemed to be greatest at 265 nm. However, conventional UV has its maximum intensity at 254 nm. Furthermore, the intensity degrades as a function of temperature and distance. This was due to the conventional tubes being designed as long, straight lamps.
The following prior art reflects the state of the art of which applicant is aware and is included herewith to discharge applicant's acknowledged duty to disclose relevant prior art. It is stipulated, however, that none of these references teach singly nor render obvious when considered in any conceivable combination the nexus of the instant invention as disclosed in greater detail hereinafter and as particularly claimed.